A seismic survey represents an attempt to image or map the subsurface of the earth by sending sound energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth. “Echoes” of that signal are then recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2-D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3-D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2-D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3-D survey produces a data “cube” or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area. In reality, though, both 2-D and 3-D surveys interrogate some volume of earth lying beneath the area covered by the survey. Finally, a 4-D (or time-lapse) survey is one that is recorded over the same area at two or more different times. Obviously, if successive images of the subsurface are compared any changes that are observed (assuming differences in the source signature, receivers, recorders, ambient noise conditions, etc., are accounted for) will be attributable to changes in the subsurface.
A seismic survey is composed of a very large number of individual seismic recordings or traces. The digital samples in seismic data traces are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Typical trace lengths when conventional impulsive sources are used are 5-16 seconds, which corresponds to 2500-8000 samples at a 2-millisecond interval. If a non-impulsive source is used, the extended activation time of the source needs to be accommodated for, so the trace lengths will generally be longer. Conventionally each trace records one seismic source activation, so there is one trace for each live source location-receiver activation. In some instances, multiple physical sources might be activated simultaneously but the composite source signal will be referred to as a “source” herein, whether generated by one or many physical sources.
In a typical 2-D survey, there will usually be several tens of thousands of traces, whereas in a 3-D survey the number of individual traces may run into the multiple millions of traces.
Of course, the ultimate goal in acquiring seismic data proximate to an exploration target is to obtain an image representative of the subsurface. To that end, a variety of seismic data processing algorithms have been developed and are routinely utilized. One of the most important of these is seismic migration which is designed to correct, among other things, the arrival time and position of reflectors in the seismic section to make them more closely match the corresponding layer configuration in the subsurface. Seismic migration is especially useful in regions with complex geology, e.g., where the subsurface contains salt domes, faults, folds, etc.
Those of ordinary skill in the art will recognize that there are very large number of migration algorithms, each with its own strengths and weaknesses. However, certain kinds of noise can be troublesome depending on the algorithm that is chosen. For example, migration “swing noise”, i.e., noise that cuts across multiple horizons, can be problematic in certain areas. Attenuating or eliminating this sort of noise and other sorts of noise such as multiples, refracted waves, etc., while preserving the underlying seismic signal, is a task that is not well handled by most migration algorithms.
As is well known in the seismic acquisition and processing arts, there has been a need for a system and method that provides a better way to migrate seismic data that does not suffer from the disadvantages of the prior art. Accordingly, it should now be recognized, as was recognized by the present inventor, that there exists, and has existed for some time, a very real need for a method of seismic data processing that would address and solve the above-described problems.
Before proceeding to a description of the present disclosure, however, it should be noted and remembered that the description which follows, together with the accompanying drawings, should not be construed as limiting the disclosure to the examples (or embodiments) shown and described. This is so because those skilled in the art to which the disclosure pertains will be able to devise other forms within the ambit of the appended claims.